Maintaining Integrity: Effects of SIP Sterilization on PTFE Valve Diaphragms

Because older SIP systems currently in use are not automated, exposure times will be longer, as manual checks and temperature mapping are performed by operators [6]. In overkill SIP cycles for large industrial fermentation systems with complex feed-system piping, steam temperature of 150°C and exposure times of four hours are needed to insure sterility [7]. The extended exposure times are known to have adverse effects on elastomeric tubing, gaskets, hoses and other polymeric materials and components [8]. The new trend of HTST sterilization shortens the hold time but exposes the equipment and diaphragms to temperatures of 140°C to 160°C [5]. As noted, high-temperature SIP cycles, typically with fast air or water cooling, also adversely affect PTFE diaphragms.

After SIP sterilization, the system is cooled down to ambient temperature and atmospheric pressure. Different aseptic systems are cooled down at different rates. Cooling with air can be fast or slow, depending upon air pressure, flow rate and complexity of the aseptic system. In a typical SIP system, air cooling takes about an hour. Cold water flush lowers the system temperature more rapidly, which is extremely damaging to the PTFE diaphragms.

During an SIP sterilization cycle, not all of the valves in a system are heated and cooled at the same time. This means that the PTFE diaphragms will be exposed to different SIP conditions impacting their integrity and performance. Valves near the cold spot will heat up and cool at slower rates.

Valves upstream and close to the steam inlet will heat up much more quickly, subjecting the PTFE diaphragms to longer, higher temperature steam exposure. Valves in large and complex systems have similar issues during the cool-down process. Upstream valves will cool down at a faster rate which is extremely detrimental to PTFE diaphragms.

SIP sterilization of large, complex systems is further complicated by uninsulated equipment and piping, which result in heat loss and temperature nonuniformities. To compensate for radiant heat losses, system operators use higher steam pressures and longer SIP cycles [9].

At the beginning of an SIP cycle, the sudden injection of steam at high velocity and pressure reduction will expand into vessels, valves and piping systems, creating a superheated steam condition [10-14]. This condition is to be avoided, since superheated steam does not conform to fixed temperature-pressure relationships; nor does it have the same “kill” kinetics for micro-organisms and spores. A clear, colorless gas, superheated steam has a greater permeability effect on PTFE diaphragms, particularly those upstream near the site of steam injection in HTST SIP cycles. It is also known to cause failure of hoses and other polymeric components.

Increased product throughput and frequent batch changes are subjecting PTFE diaphragms to greater frequency and higher temperature SIP sterilization with fast cooling. As a result, diaphragm failures, including bead deformation, leakage, and obstruction of flow due to diaphragm warpage and distortion, are becoming more frequent. And recently there also have been reports of diaphragm blistering, in which crevices and other surface anomalies form, putting at risk the sterility of aseptic systems.

Blistering in PTFE Diaphragms
A number of field cases of PTFE diaphragm blistering have been reported after SIP sterilization. The diaphragms were used in sanitary systems in pharmaceutical/bioprocessing, food and dairy facilities. The blistered diaphragms were provided by the valve manufacturers after they were returned by end-users. Following are three examples of PTFE diaphragms that developed blisters during SIP sterilization at higher than typical temperatures.

Case 1: Figure 3 shows a blistered DIN 20 weir-type PTFE diaphragm that was SIP sterilized at 150°C. The blisters were discovered during a scheduled maintenance interval, and when pierced expelled a clear, water-like liquid. The PTFE diaphragm also sustained external bead deformation, flex fatigue stresses and permeation damage, as evidenced by the cloudy discolorations.

Figure 3. Blistered PTFE Diaphragm

Case 2: The PTFE valve diaphragm underwent ~140°C high-temperature SIP once a day, and was cleaned in place with caustic soda and nitric acid. Again blisters containing a clear liquid were detected during system maintenance. The blisters were the same in appearance as the blisters in Figure 3. Analyzed with Fourier transform infrared (FTIR) spectroscopy, the liquid was a near match with distilled water, and its moderately acidic ~3 pH was consistent with the nitric acid cleaning.

Case 3: Sanitary valves with PTFE diaphragms were SIP sterilized once a day at 145°C with exposure time of 20 minutes, and cooled down with a water quench. The valves underwent total of 50,000 actuations and ~400 SIP cycles, after which blisters with the same appearance and morphology as those in Case 1 were discovered during system maintenance. These blisters also contained clear liquid.

Testing to Replicate Blistering
To investigate the effects of SIP sterilization on PTFE valve diaphragms, laboratory equipment was designed and built to test standard DIN 50 (2") weir-type valves with automatic actuators and assembled with sanitary stainless steel piping with tri-clamp connections (Figure 4). The test equipment was connected to industrial saturated stem for SIP, clean potable water for cooling, and plant air for purge. This equipment was fully interfaced with a PC control system for automated operation, data collection and SIP cycle control.

Figure 4. Test Equipment for Sanitary Diaphragm Valves